12sj7 pedal

[ampedup]

http://s1016.photobucket.com/user/v8nutz/media/IMG_0576_zpslwuebuso.jpg.html
I built the clean boost version of this 12sj7 pedal some years ago and I like it very much, it was posted on the ax84 forum by david jones. He also posted this version which adds an extra 12ax7 gain stage with tone controls. So I crammed the other tube and controls in there and it works but with a couple of issues. The gain control is really sensitive, would going to a 500k instead of 1meg make it better? Also the bass control doesn't seem to work, it acts more like a volume at one end of the turn. Any one see any problem areas on the schematic? Would the tone controls act this way if I wired them backwards?

[GibsonGM]

Well, amped...I think if you look at your build, compared to the way it's laid out...if you built it like the person designed it, it should be OK.  When you "crammed" the mod version together, did you follow the schematic as it was written?   I don't think he'd release a design with problems like you are having...usually it's some mistake WE make

[mth5044]

You might seitch the gain pot taper

[ampedup]

Looking at the back of the pots I treated the left as ground side, was that right?

[GibsonGM]

If you're saying what I think you are, no - the R side of the pot, from the back, would be the 'cold' side, leading to ground.   

If you think about it a bit more, you might be able to see that your way, you'd be turning the pot to the R from the front, making the wiper to ground resistance lower.  That means you're making the wiper to hot side resistance HIGHER, lowering the signal level, making 'righty = quieter'. The pot is like 2 resistances in series that balance - one goes up, the other goes down...so there's one problem hopefully fixed! 

[mth5044]

By taper I meant log or linear, A or B. Perhaps you used a log pot when it was supposed to be linear or vice versa. Could cause the 'bunching'.

[ampedup]

If your right I wired all my pots backwards!

[ampedup]

I just looked at some  layouts over at amp garage and the pots always go to ground on the left looking from the back.

[davent]

I just looked at some  layouts over at amp garage and the pots always go to ground on the left looking from the back.

Is that with the lugs pointing skyward or to earth?

[teemuk]

The gain control is really sensitive, would going to a 500k instead of 1meg make it better? Also the bass control doesn't seem to work, it acts more like a volume at one end of the turn. Any one see any problem areas on the schematic? Would the tone controls act this way if I wired them backwards?

Wiring them backwards would simply make the controls operate backwards.

e.g. If you wired a volume control backwards, volume would not increase when you turn the dial clockwise, it would decrease. Turning the dial counter clockwise would increase volume. Simply "backwards" operation.

Wiring them backwards would have no effect on "taper" of the control and therefore no effects on overall "sensitivity" of the control or how consistently the control works throughout the whole dial range. Controls working effectively only at very narrow range of the dial sound more like tapering issues.

For example, your "gain" control pot: Big difference whether its taper is linear or logarithmic. You have also modified the taper  by wiring a 470K resistor in parallel with potentiometer's wiper and ground. The effect of this resistance will often be significant in the resistive voltage divider, which is your gain control.

This effect will also, by a good chance, be a horrible "tone sucker". It's output impedance is output Z of the tube stage + Z of the tonestack signal path + Z of resistive track from pin 3 to wiper of the 1M volume pot in the output + 2K2 (which after all this series resistance seems like a futile addition). I estimate you land in about 1M output impedance or above. Plug this thing into even any generic amp or effect and the effect of such very high output Z will attenuate the signal to about 1/2. And it's most likely not happening linearly throughout the effective bandwidth.... And add to the insult, the controls will interactively vary output impedance. Output Z should preferably remain rather constant and be in the order of 10K so you're couple -decades- off from ideal. Not good design.

[ampedup]

do you have any suggestions to improve this?

[ampedup]

is there a better bass treble circuit I could use instead?

[PRR]

> Any one see any problem areas on the schematic?

No.

I suggest that it isn't built as designed.